Method for inspecting optical information recording medium, inspection apparatus, optical information recording medium and recording method

ABSTRACT

A method for inspecting an optical information storage medium includes irradiating the medium with a laser beam and rotating the medium by a constant linear velocity control technique by reference to the radial location at which the laser beam forms a spot on the medium; changing the rotational velocities according to the radial location on the medium between at least two linear velocities; generating a focus error signal and/or a tracking error signal based on the light reflected from the medium; performing a focus control and/or a tracking control on the laser beam based on the focus and/or tracking error signal(s); and passing the branched outputs of control loops for the focus and/or tracking error signal(s) through predetermined types of frequency band-elimination filters to obtain residual errors of the focus and/or tracking error signal(s) and comparing the residual errors to predetermined reference values.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for inspectingan optical information storage medium, an optical information storagemedium, and a method of writing information on such a medium. Moreparticularly, the present invention relates to a method and apparatusfor inspecting an optical information storage medium on which aread/write operation needs to be performed at high rates and alsorelates to a method for inspecting such an optical information storagemedium for residual focus and tracking errors.

2. Description of the Related Art

An optical information storage medium has a storage layer on whichinformation is written as pits or marks. That information can be read byirradiating the pits or marks with light and by detecting a variation inthe intensity of the light reflected. Such an optical informationstorage medium normally has a disc shape, and therefore, is called an“optical disc”. Thus, according to that normal practice, an opticalinformation storage medium will be simply referred to herein as an“optical disc”.

Nowadays, Blu-ray discs (BDs), digital versatile discs (DVDs) and otheroptical discs with high densities and big storage capacities have becomeincreasing popular and have been used more and more extensively to storecomputer data, software, audiovisual data and so on.

Among those optical discs with high densities and big storagecapacities, there are increasing demands on the market for write-oncediscs such as DVD-Rs and BD-Rs, in particular. A write-once optical discmay have a storage layer including a Te—O-M based material (where M isat least one element selected from the group consisting of metallicelements, metalloid elements and semiconductor elements) as disclosed inJapanese Patent Application Laid-Open Publication No. 2004-362748. TheTe—O-M based material is a compound material, which includes Te, O and Mand in which fine particles of Te, Te-M and M are randomly dispersed ina TeO₂ matrix of the as-deposited material. When the storage layer ofsuch a material is irradiated with a laser beam with at least apredetermined intensity, the portion of the storage layer irradiatedwith the laser beam will melt to precipitate Te or Te-M crystals withlarge particle sizes while being cooling, thereby forming a recordingmark on the storage layer. That portion where the crystals have beenprecipitated has a different optical property from the other portions.That is why when the recording mark is irradiated with a laser beam, adifference will be made on the intensity of the reflected light and thedifference in the intensity of the reflected light can be detected as asignal. In this manner, a so-called “write-once operation”, which allowsthe user to perform a write operation only once, can get done.

The rotational velocity of an optical disc can be controlled by a CLV(constant linear velocity) technique or a CAV (constant angularvelocity) technique. Specifically, according to the CLV controltechnique, the rotational frequency of a given optical disc iscontrolled inversely proportional to the radial location and informationis supposed to be written in response to a certain number of writechannel clock pulses while making the scanning light beam follow thetracks at a constant linear velocity. On the other hand, according tothe CAV control technique, the rotational frequency is kept constanteven while data is being written on the optical disc but channel clockpulses are applied during writing as a reference signal to the opticaldisc at variable frequencies that are proportional to the radiallocation of the scanning light beam on the tracks. In that case, channelclock pulses are applied at low frequencies on the inside portion of thedisc but are applied at high frequencies on the outside portion of thedisc. Then the recording linear velocity will be low on the insideportion and high on the outside portion but recording marks will be leftwith a constant recording linear density.

In writing information on an optical disc or reading the informationstored there from the disc, the optical disc needs to be irradiated witha laser beam that has been converged in a predetermined state. In such asituation, a type of control to be performed by an optical disc drive tokeep the laser beam in such a predetermined converged state is called a“focus servo control”, while another type of control to be performed bythe optical disc drive to move the laser beam spot in the disc radialdirection so as to follow the tracks, which are a series of marks lefton the storage layer, is called a “tracking servo control”. Also, asignal representing the magnitude of shift from the predeterminedconverged state of the laser beam in the focus servo control is called a“focus error signal”. Likewise, a signal representing the magnitude ofdeviation of the laser beam from the target tracks in the tracking servocontrol is called a “tracking error signal”. The tracking error issometimes called a “radial tracking error” and the focus error issometimes called an “axial tracking error”.

For example, Japanese Patent Application Laid-Open Publication No.2004-5817 and Japanese Patent Publication No. 3819138 disclosetechnologies relating to focus and tracking servo controls to beperformed on a write-once optical disc. These documents disclose anoptical disc drive and method for performing write processing with highreliability by controlling the write rate based on the focus errorsignal and other signals and a method for detecting the values ofvibrations to be produced due to the eccentricity of the disc based onthe tracking error signal.

Recently, particularly in computer peripheral devices and optical discrecorders that are compatible with optical discs with huge storagecapacities, it is more necessary to get a write operation done at hightransfer rates than anything else. Specifically, there is an increasingdemand for development of a technique for reading or writing informationat rates corresponding to 6× velocity for BDs. To achieve such hightransfer rates, however, the optical disc should be scanned with a laserbeam much more quickly by increasing the rotational frequency (or thelinear velocity) of the disc. As used herein, the “**×velocity”, forexample, means that the velocity is ** times as high as the standardread/write rate. More specifically, the read/write rate is representedas either a linear velocity or a transfer rate. In this description, theread/write rate will be represented herein by the linear velocity inmost cases.

Generally speaking, however, if the rotational frequency of a disc wereincreased, then the locations on the tracks where the information iswritten and the levels (i.e., heights) of the storage layer would changequickly due to out-of-plane vibrations, eccentricity, defects,variations in thickness distribution and other shape imperfections ofthe optical disc. Thus, the focus servo control and the tracking servocontrol should be performed even more quickly. However, there is acertain limit to the response of the servo control. That is why if theon-track locations or the levels of the storage layer changed atfrequencies that are even higher than the quickest possible response ofthe servo control, then it would be impossible for the optical discdrive to get the focus servo control or the tracking servo control doneperfectly. As a result, the tracking error signal would have anincreased residual error (which will be referred to herein as a“residual tracking error”), thus decreasing the stability of thetracking servo. And the residual error of a focus error signal (whichwill be referred to herein as a “residual focus error”) would alsoincrease and the envelope of a write signal would have missing (orzero-amplitude) portions corresponding to the residual error to possiblydecrease the symbol error rate (SER) significantly.

As used herein, the “residual tracking error” refers to a signalcomponent to be produced in a situation where the tracking control hasnot been done quite successfully. That is to say, even if the opticaldisc drive is performing a tracking servo control appropriately enough,the laser beam may still be unable to follow the tracks perfectly tomake the level of the tracking error signal not equal to zero, which iswhat is called a “residual tracking error”. Likewise, the “residualfocus error” refers to a signal component to be produced in a situationwhere the focus control has not been done quite successfully. That is tosay, even if the optical disc drive is performing a focus servo controlappropriately enough, the laser beam may still deviate from thepredetermined converged state to make the level of the focus errorsignal not equal to zero, which is what is called “residual focuserror”. The residual error of each of these signals is estimated by theamplitude of that signal. And the optical disc drive represents thevalues of those residual errors by the magnitude of deviation of thelaser beam spot from the center of the tracks and by that of shift ofthe focal point of the laser beam from the target storage layer,respectively. More specifically, these magnitudes are represented asdistances (or lengths). That is why the tracking error signal may berepresented as having a residual error of xx nm and the focus errorsignal may be represented as having a residual error of xx nm. It shouldbe noted that the residual errors are sometimes called simply“residuals”. In this description, when just “residual errors” arementioned, the residual errors refer to both the residual tracking errorand the residual focus error alike.

For these reasons, it is necessary to control the shape of a stamper tobe used as a master to make an optical disc, the forming process of theoptical disc, the viscosity of the resin material of its coating layer,and the thickness of a spin-coated film with even higher degrees ofprecision. Added to that, it is no less important to develop aninspecting method and apparatus that can efficiently and preciselydetermine whether or not the optical disc product just made has expectedshape precision or mechanical properties.

However, if the spindle motor of such an inspecting apparatus carriedout the inspection while rotating at six times as high velocities asnormal BDs, then significant residual focus and tracking errors would bedetected from mechanical factors of the inspecting apparatus itself,e.g., vibrations and resonance of the actuator. Then, it would beimpossible to precisely measure the residual errors that have beencaused due to the mechanical properties of the optical disc (or get theinspection done) just as originally intended. Nevertheless, if anexpensive high-performance inspecting apparatus that would have reducedvibrations or actuator resonance were newly introduced, then investmenton equipment should be newly made, thus eventually increasing themanufacturing cost of the media.

Also, if a write operation were performed by the CLV control techniqueon the entire surface of an optical disc at as high a linear velocity as6× rate for BDs, then the rotational frequency of the spindle motorshould be higher than 10,000 rpm on the inside portion of the disc. Thisis a problem because 10,000 rpm is the maximum allowable rotationalfrequency in practice that was determined from safety considerations inview of the rupture limit of plastic that is the substrate material ofthe disc. For that reason, the optical disc should not be inspected atsuch a high velocity as exceeding 10,000 rpm.

Furthermore, the residual errors of the tracking error signal or thefocus error signal could be reduced by performing the servo controlswith higher precision with the servo filter characteristic of theinspecting apparatus adjusted. However, an optical disc drive thatperforms a write operation on BDs at 4× linear velocity performs focusand tracking servo control operations using a servo filter that alreadyhas as high a gain intersection as 6 kHz to 8 kHz. For that reason, ifthe servo characteristic of the inspecting apparatus should have an evenhigher gain intersection to cope with the 6× linear velocity for BDs,then the actuator would have a decreased oscillation or phase margin,thus making it virtually impossible to secure servo stability.

In order to overcome the problems described above, the present inventionhas an object of providing a method and apparatus for preciselyinspecting an optical information storage medium, on which a read/writeoperation should be performed at high linear velocities. Another objectof the present invention is to provide a method of writing a signal ofquality on such an optical information storage medium. Still anotherobject of the present invention is to provide such an opticalinformation storage medium.

SUMMARY OF THE INVENTION

A method for inspecting an optical information storage medium accordingto the present invention includes the steps of: irradiating the opticalinformation storage medium with a laser beam and rotating the storagemedium by a constant linear velocity control technique by reference tothe radial location at which the laser beam forms a spot on the storagemedium; changing the rotational velocities according to the radiallocation on the storage medium between at least two linear velocitiesthat include a first linear velocity Lv1 and a second linear velocityLv2, which is higher than the first linear velocity Lv1; generating afocus error signal and/or a tracking error signal based on the lightreflected from the storage medium; performing a focus control and/or atracking control on the laser beam that irradiates the storage mediumbased on the focus error signal and/or the tracking error signal; andpassing the branched outputs of control loops for the focus error signaland/or the tracking error signal through predetermined types offrequency band-elimination filters for the focus and/or tracking errorsignal(s) to obtain residual errors of the focus and/or tracking errorsignal(s) and comparing the residual errors to predetermined referencevalues.

In one preferred embodiment, the comparison is made by rotating theoptical information storage medium at the first linear velocity Lv1 ator inside of a predetermined radial location R on the storage medium butat the second linear velocity Lv2 outside of the predetermined radiallocation R on the storage medium.

In another preferred embodiment, the Lv2/Lv1 ratio of the second linearvelocity Lv2 to the first linear velocity Lv1 is either 1.5 or 2.

In still another preferred embodiment, the first linear velocity Lv1 isa positive real number of times as high as 9.834 m/sec or 4.917 m/secand/or the second linear velocity Lv2 is a positive real number of timesas high as 14.751 m/sec or 4.917 m/sec.

In yet another preferred embodiment, if Lv2/Lv1=1.5, the predeterminedradial location R satisfies 33 mm≦R≦36 mm but if Lv2/Lv1=2.0, thepredetermined radial location R satisfies 44 mm≦R≦48 mm.

In yet another preferred embodiment, each of the first and second linearvelocities is a half or less as high as the maximum one of linearvelocities for reading and/or writing that are stored in advance in apredetermined area of the optical information storage medium.

In yet another preferred embodiment, the gain intersection of the servocharacteristic of the focus control remains the same, no matter whetherthe optical information storage medium, being subjected to the focuscontrol to make a comparison to the predetermined reference value, isrotated at the first linear velocity or the second linear velocity. Thegain intersection of the servo characteristic of the tracking controlalso remains the same, no matter whether the optical information storagemedium, being subjected to the tracking control to make a comparison tothe predetermined reference value, is rotated at the first linearvelocity or the second linear velocity.

In yet another preferred embodiment, the frequency band-eliminationfilter for the focus error signal includes a low-pass filter LPF with acutoff frequency LPF_FcL and a band-pass filter BPF with a lower cutofffrequency BPF_FcL and a higher cutoff frequency BPF_FcH. The branchedoutput of the control loop for the focus error signal is supplied to thelow-pass filter LPF and the band-pass filter BPF. If the opticalinformation storage medium is rotated at the first and second linearvelocities and subjected to the focus control to make a comparison tothe predetermined reference value, LPF_FcL, BPF_FcL and BPF_FcH areswitched one after another according to the ratio of the second linearvelocity to the first linear velocity.

In this particular preferred embodiment, the frequency band-eliminationfilter for the tracking error signal includes a low-pass filter LPF witha cutoff frequency LPF_TcL and a band-pass filter BPF with a lowercutoff frequency BPF_TcL and a higher cutoff frequency BPF_TcH. Thebranched output of the control loop for the tracking error signal issupplied to the low-pass filter LPF and the band-pass filter BPF. Thecutoff frequencies LPF_TcL and BPF_TcL are constant irrespective of thefirst and second linear velocities. And BPF_FcH is switched one afteranother according to the ratio of the second linear velocity to thefirst linear velocity.

In a specific preferred embodiment, the output value F_LPF of the focuserror signal that has been passed through the LPF, the output valueF_BPF of the focus error signal that has been passed through the BPF,the output value T_LPF of the tracking error signal that has been passedthrough the LPF, and the output value T_BPF of the tracking error signalthat has been passed through the BPF are all compared to theirassociated predetermined reference values.

In a more specific preferred embodiment, when compared to thepredetermined reference values according to the radial location, thefour output values F_LPF, F_BPF, T_LPF and T_BFP are compared to twosets of reference values that are defined for the first and secondlinear velocities, respectively.

In this particular preferred embodiment, the reference value for F_LPFat the second linear velocity is equal to or greater than the referencevalue for F_LPF at the first linear velocity.

In yet another preferred embodiment, the intensity of the laser beamremains the same irrespective of the linear velocity.

An optical information storage medium according to the present inventionis designed to read and/or write information optically from/on it. Whenthe optical information storage medium is subjected to a predeterminedinspection with information about a velocity that is k times (where k isa positive real number) as high as a standard read/write rate on theoptical information storage medium stored as velocity information in apredetermined area on the optical information storage medium, thestorage medium is inspected at a first measuring rate in a first radialrange on the optical information storage medium. But the storage mediumis inspected at a second measuring rate in a second radial range that islocated outside of the first radial range.

In one preferred embodiment, the second measuring rate is less than ktimes as high as the standard rate, and the first measuring rate islower than the second measuring rate.

In another preferred embodiment, k is a positive real number that isequal to or greater than six.

Another optical information storage medium according to the presentinvention is also designed to read and/or write information opticallyfrom/on it. If the storage medium is a first type of optical informationstorage medium on which information about a velocity that is m times(where m is a positive real number) as high as a standard read/writerate on the optical information storage medium is stored as velocityinformation in a predetermined area on the optical information storagemedium, then the first type of optical information storage medium isinspected at a predetermined measuring rate. But if the storage mediumis a second type of optical information storage medium on whichinformation about a velocity that is n times (where n is a positive realnumber that is greater than m) as high as the standard read/write rateon the optical information storage medium is stored in a predeterminedarea on the optical information storage medium, then the second type ofoptical information storage medium is inspected with the measuring rateschanged according to the radial location on the second type of opticalinformation storage medium.

In one preferred embodiment, one of the measuring rates on the secondtype of optical information storage medium is higher than the measuringrate on the first type of optical information storage medium. The othermeasuring rate on the second type of optical information storage mediumis equal to or higher than the measuring rate on the first type ofoptical information storage medium.

In a specific preferred embodiment, m is a positive real number that isequal to or greater than four and/or n is a positive real number that isequal to or greater than six.

A reading method according to the present invention is a method ofreading information from an optical information storage medium accordingto any of the preferred embodiments of the present invention describedabove. The method includes the steps of: irradiating the opticalinformation storage medium with light; and reading the velocityinformation from the predetermined area on the storage medium.

According to the present invention, the residual error(s) of a focuserror signal and/or a tracking error signal are/is measured with therotational velocities changed according to the radial location on theoptical information storage medium between at least two linearvelocities that include a first linear velocity Lv1 and a second linearvelocity Lv2 that is higher than the first linear velocity Lv1. Bychanging the rotational velocities, the linear velocity, and therotational velocity of the optical information storage medium, can bedecreased on an inner area of the storage medium. That is why it ispossible to prevent the mechanical factors of the inspecting apparatusitself (such as the vibrations of the inspecting apparatus and theresonance of the actuator) from affecting the residual error(s) of thefocus error signal and/or the tracking error signal even in an opticalinformation storage medium from/on which information needs to be read orwritten at high rates. As a result, the residual errors resulting fromthe mechanical properties of the given optical information storagemedium can be measured precisely.

Thus, the present invention provides an inspecting method thatcontributes to sorting out an optical information storage medium ofquality that ensures a read signal of quality (i.e., with a good SER)and tracking servo stability. The present invention achieves this objectby preventing various types of disturbances such as out-of-planevibrations, eccentricity, defects and variation in thicknessdistribution from increasing the residual error components of thetracking error signal so much as to affect the servo stability with atracking servo failure or the actuator's oscillation while a read/writeoperation is being performed on the optical information storage medium.The present invention also prevents the residual error components of thefocus error signal from increasing so much as to cause the envelope of awrite signal to have any missing portion due to the residual error anddecreasing the SER of the read signal significantly. The presentinvention can be used particularly effectively to inspect a write-onceor rewritable optical disc on which information can be written at ashigh a linear velocity as 6× rate for BDs (with a channel clockfrequency of 396 MHz).

On top of that, according to the present invention, at least one of thehighest writable linear velocity and radial location information iswritten on a predetermined area of the optical information storagemedium, thereby making it possible to use the same residual errorproperty inspecting apparatus during the manufacturing process ofoptical information storage media. As a result, the equipment cost canbe minimized, the production yield of the media can be increased, andeventually the manufacturing cost of the optical information storagemedia can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a format for an optical information storage mediumaccording to a preferred embodiment of the present invention.

FIG. 2 shows how the rotational velocity changes according to the radiallocation in a situation where a read/write operation is performed on anoptical information storage medium in a 6×CLV mode in a preferredembodiment of the present invention.

FIG. 3 shows how the rotational velocity changes according to the radiallocation in a situation where a read/write operation is performed on anoptical information storage medium in 4×CLV mode and then in 6×CLV modein a preferred embodiment of the present invention.

FIG. 4 shows the relations between the radial location and therotational velocity in a situation where a read/write operation isperformed on an optical information storage medium by the CLV controltechnique with the linear velocities changed between 4×, 6× and 8×according to the radial location in a preferred embodiment of thepresent invention.

FIG. 5 illustrates an exemplary overall configuration for an opticalinformation storage medium inspecting apparatus according to a preferredembodiment of the present invention.

FIG. 6 schematically shows the servo gain characteristic of theinspecting apparatus shown in FIG. 5.

FIG. 7 is a block diagram illustrating a residual tracking errormeasuring section of the inspecting apparatus shown in FIG. 5.

FIG. 8 is a block diagram illustrating a residual focus error measuringsection of the inspecting apparatus shown in FIG. 5.

FIG. 9 shows the characteristics of measuring filters for use in theresidual tracking and focus error measuring sections shown in FIGS. 6and 7.

FIG. 10A shows the residual focus errors that were measured when thedisc was rotated at 4× linear velocity.

FIG. 10B shows the residual focus errors that were measured when thedisc was rotated at 2× linear velocity.

FIG. 11 shows what relation the RF signal waveform and the residualfocus error will have in a situation where a write operation isperformed while there is a significant residual focus error.

FIG. 12 shows how the probability of tracking failures changes with theresidual tracking error according to the frequency of disturbance.

FIG. 13 shows the relations between the residual focus errors and theirdefocus margins.

FIG. 14 shows the relations between the radial location and therotational velocity in a situation where a read/write operation isperformed with the modes of control operations changed from 4×CAV into6×CLV according to the radial location in a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a format for an optical information storage mediumhaving a data area 1001 and a PIC zone 1003, according to a preferredembodiment of the present invention. FIG. 2 shows the relation betweenthe write location and the rotational velocity in a situation whereinformation needs to be read or written from/on a BD-R at 6x linearvelocity. The write location is indicated by the radius r. In that case,the BD-R should be rotated at a rotational velocity of approximately12,000 rpm in the innermost portion (where r=24 mm) of the data area1001, and at a rotational frequency of approximately 4,800 rpm in theoutermost portion of the data area 1001. As can be seen from FIG. 2,however, if a read/write operation is performed on an inner range wherethe radius r is approximately 28 mm or less, the rotational velocity ofthe spindle motor exceeds 10,000 rpm.

As described above, the rotational velocity of the optical disc shouldnot be higher than 10,000 rpm considering the rupture limit of plastic.For that reason, the optical disc should not be inspected at such a highvelocity as exceeding 10,000 rpm, either. Also, at such a high velocity,the servo characteristic of the inspecting apparatus might lose itsstability and the optical disc could not be inspected accurately. Thatis why according to the present invention, the optical disc is supposedto be inspected with a proper limit imposed on the highest rotationalvelocity.

Specifically, the linear velocity on the inner portion of the disc isset to be lower than on the outer portion thereof so that the highestrotational velocity defined is not exceeded. FIG. 3 shows the relationsbetween the radial location and the rotational velocity of the opticaldisc in a situation where a read/write operation is performed by the CLVcontrol technique at 4× linear velocity on an inner area defined byradial locations r of approximately 36 mm or less and at 6× linearvelocity on the remaining area that is located outside of the innerarea. In that case, the highest rotational velocity at 4× linearvelocity in the innermost portion (where r=24 mm) of the data area 1001is approximately 8,000 rpm. That is why if the highest rotationalvelocity over the entire surface of the optical disc is set to be thisvalue, then the rotational velocity will be approximately 8,000 rpm at aradial location r of approximately 36 mm when the linear velocity is 6×.Therefore, by performing a read/write operation at the 4× linearvelocity inside of the radial location r of 36 mm but at the 6× linearvelocity at or outside of the radial location r of 36 mm, the rotationalvelocity of the optical disc can always be kept equal or lower thanapproximately 8,000 rpm.

In this manner, by switching the two linear velocities at the radiallocation of 36 mm, which is 1.5 times (that is the ratio of the 6×linear velocity to the 4× linear velocity) as distant from the center asthe innermost radial location of 24 mm, the highest rotationalvelocities can always be equal to each other, no matter whether the discis rotated at the 4× linear velocity or at the 6× linear velocity. Byperforming a read/write operation with the upper limit set to thehighest rotational velocity and with the linear velocities changedaccording to the radial location, even if the linear velocities aredifferent but if the mutually different linear velocities have the samehighest rotational velocity, then there is no need to modify thelow-frequency gain characteristic of a servo filter (reference servo)among various servo characteristics such as tracking and focus servocontrols according to the highest rotational velocity of each linearvelocity in order to catch up with the variation in eccentricity orout-of-plane vibrations of the disc. Then the servo filter of theoptical disc drive may have the same characteristic, no matter whetherthe linear velocity is 4× or 6×.

On top of that, the servo filter of an apparatus for inspecting anoptical disc for residual errors (to be described later) can alsomaintain the same characteristic. That is to say, there is no longer anyneed to suspend the tracking or focus control operation, modify thesettings of the servo filter (reference servo) and then resume thetracking or focus control operation and read operation again in order toswitch or change the servo filters according to the linear velocity. Asa result, the inspection can get done in a shorter time.

Added to that, when the residual errors are measured at multipledifferent linear velocities, the entire storage area of the disc can beinspected continuously (i.e., from the innermost portion through theoutermost portion thereof) for residual errors just by changing themeasuring rotational velocities and the cutoff frequencies of theresidual error measuring filter (to be described later). Consequently,the inspection can get done in a much shorter time. As a result, thetact time can be shortened and the productivity of optical discs can beincreased. Furthermore, the residual error measuring and inspectionprocesses can be carried out under the same reference servo conditionsas an inspecting apparatus for 4× BD-R discs. That is to say, a residualerror inspecting apparatus for 4× BD-R discs can be used as it is toinspect a 6× BD-R, too. By combining the respective lines together inthis manner, there is no need to introduce a new inspecting apparatus,thus cutting down the equipment cost significantly. As a result, a hugenumber of media can be mass-produced at a much lower cost.

In the example described above, the radial location r to switch thelinear velocities is supposed to be 36 mm. However, this is just anexample in a situation where the ratio of the linear velocities is 1.5.Alternatively, if the innermost radial location of 22.2 mm in thelead-in zone is supposed to be a reference radial location and if thehighest rotational velocity at that radial location is regarded as theupper limit of the rotational velocity (which is approximately equal to8,000 rpm), then the 4× and 6× linear velocities may be switched oneafter the other at a radial location of 33.3 mm. Still alternatively,the innermost radial location of 22.7 mm in the OPC zone on Layer 1 of adual-layer disc may also be defined as a reference radial location. Thatis to say, it is appropriate to set the radial location to switch thelinear velocities within the range of approximately 33 mm toapproximately 36 mm.

Alternatively, if 4× linear velocity and 8× linear velocity are adopted,then the ratio Lv2/Lv1 of linear velocities is 2.0. In that case, if therotational velocity at the radial location of 22.2 mm to 24 mm issupposed to be the upper limit of the rotational velocity as in 6× evenwhen a read/write operation needs to be performed at the lower linearvelocity (i.e., 4× linear velocity), then it is appropriate to set theswitching radial location within the range of approximately 44 mm toapproximately 48 mm.

Optionally, a read/write operation may also be performed on a singleoptical disc at three or more linear velocities. In that case, thenumber of radial locations to switch the linear velocities becomessmaller than that of linear velocities to use by one. For example, toperform a read/write operation at 4×, 6× and 8× linear velocities, tworadial locations to switch the linear velocities need to be defined.FIG. 4 shows the relations between the radial location and therotational velocity in a situation where a read/write operation isperformed by the CLV control technique with the linear velocitieschanged between 4×, 6× and 8×. The upper limit of the rotationalvelocities for the respective linear velocities is defined by therotational velocity at the innermost reference radial location for the4× linear velocity. The ratios of the 6× and 8× linear velocities withrespect to the reference linear velocity of 4× become 1.5 and 2.0,respectively. That is why if a first radial location, which defines areference radial location of the innermost area of the disc for the 4×linear velocity, is set within the range of 22.2 mm to 24 mm, the secondand third switching radial locations may be set within the range ofapproximately 33-36 mm and within the range of approximately 44-48 mm.Then, a read/write operation may be performed at 4× linear velocityinside of the first radial location, at 6× linear velocity between thefirst and second radial locations, and at 8× linear velocity outside ofthe second radial location. In that case, the highest rotationalvelocity will always be approximately 8,000 rpm, which is the rotationalvelocity at the first radial location that defines the reference radiallocation in the innermost area for the 4× linear velocity, no matterwhich of these three linear velocities is used.

Then, compared to a situation where the linear velocities are switchedbetween 4× and 8×, a read/write operation can be performed at 6× linearvelocity, not 4× linear velocity, between the first and second switchingradial locations. As a result, the read/write rate on the overalloptical disc can be increased and the read/write time can be shortened.

Hereinafter, a preferred embodiment of an optical information storagemedium inspecting apparatus according to the present invention will bedescribed. The optical information storage medium inspecting apparatusof the preferred embodiment to be described below measures the residualtracking error of a tracking error signal generated and the residualfocus error of a focus error signal generated while rotating the opticaldisc to inspect at multiple different linear velocities described aboveand performing a focus control and a tracking control on it with respectto the laser beam emitted from an optical pickup. And then the apparatuscompares those residual error values obtained to reference values,thereby determining the given optical disc to be a GO or a NO-GO.

FIG. 5 is a block diagram illustrating an overall configuration for anoptical information storage medium inspecting apparatus as a preferredembodiment of the present invention. The inspecting apparatus shown inFIG. 5 is designed to inspect an optical disc 101 such as a BD-R, whichmay have the structure that has already been described with reference toFIG. 1.

The inspecting apparatus shown in FIG. 5 includes a spindle motor 102,an optical pickup 103, a laser driver section 116, a rotational velocitysetting section 117 and RF amplifiers 104, 105 and 106.

The optical disc 101 is rotated and driven by the spindle motor 102, ofwhich the rotational velocity is controlled by the rotational velocitysetting section 117. The laser driver section 116 drives a semiconductorlaser 103 a in the optical pickup 103, thereby irradiating the opticaldisc 101 with a laser beam with readout power. The light reflected fromthe optical disc 101 is transmitted through a detector lens 103 a andthen received at, and converted into an electrical signal by, aphotodetector 103 c. Then, the electrical signal is supplied to the RFamplifiers 104, 105 and 106.

The inspecting apparatus further includes a read signal processingsection 107, a tracking servo amplifier 108 and a focus servo amplifier109. The RF amplifier 104 amplifies the output of the optical pickup 103and then passes an RF signal to the read signal processing section 107.Meanwhile, the RF amplifiers 105 and 106 respectively generate atracking error (TE) signal and a focus error (FE) signal based on theoutput of the optical pickup 103, and then supply them to the trackingservo amplifier 108 and the focus servo amplifier 109, respectively.

The inspecting apparatus further includes a tracking actuator driver 110and a focus actuator driver 111. The tracking servo amplifier 108generates a control signal based on the tracking error signal andoutputs it to the tracking actuator driver 110, while the focus servoamplifier 109 generates a control signal based on the focus error signaland outputs it to the focus actuator driver 111. The tracking and focusactuator drivers 110 and 111 generate drive signals based on the controlsignals and use those signals to drive the drive coils in the trackingand focus directions in the optical pickup 103. As a result, a trackingservo control loop that uses the tracking error signal is formed by theoptical pickup 103, the RF amplifier 105, the tracking servo amplifier108 and the tracking actuator driver 110. Likewise, a focus servocontrol loop that uses the focus error signal is formed by the opticalpickup 103, the RF amplifier 106, the focus servo amplifier 109 and thefocus actuator driver 111.

FIG. 6 schematically shows the gain characteristic of servo filters foruse to perform the tracking and focus servo controls. The gaincharacteristic of the servo filters is also called a “reference servocharacteristic”. The tracking and focus servo controls have apredetermined reference servo characteristic. As shown in FIG. 6, theservo characteristic has a predetermined gain level at low frequenciesbut comes to have a decreased gain as the frequency increases. And thefrequency f0 at which the gain goes zero decibels is called a “gaincrossover frequency”. The servo characteristic is characterized mainlyby this gain crossover frequency. The servo characteristics of thetracking and focus servo controls are different from each other.However, even if the linear velocities are changed while the givenoptical disc is being inspected, the tracking and focus servo controlsare still carried out with the same servo characteristic.

The inspecting apparatus further includes a residual tracking errormeasuring section 112, a residual focus error measuring section 113, amemory 114 and a decision section 115. Part of the tracking error signalsupplied from the RF amplifier 105 is branched from the tracking errorsignal control loop and then passed to the residual tracking errormeasuring section 112. As will be described in detail later, theresidual tracking error measuring section 112 extracts a residualtracking error from the tracking signal that has been obtained byperforming the tracking servo control and outputs it to the memory 114.In the same way, part of the focus error signal supplied from the RFamplifier 106 is branched from the focus error signal control loop andthen passed to the residual focus error measuring section 113. Theresidual focus error measuring section 113 extracts a residual focuserror from the focus error signal that has been obtained by performingthe focus servo control and outputs it to the memory 114. These residualtracking and focus errors are measured at each radial location on theoptical disc.

Then, the decision section 115 compares the residual tracking and focuserror values that are now retained in the memory to the predefinedreference values of the residual tracking and focus errors, therebydetermining whether the disc in question is a GO or a NO-GO. Forexample, if both of the residual tracking and focus errors at eachradial location are equal to or smaller than their reference values, thedecision section 115 finds the optical disc inspected a GO.

FIGS. 7 and 8 illustrate configurations for the residual tracking errormeasuring section 112 and the residual focus error measuring section113, respectively. The residual tracking error measuring section 112includes a buffer 201, an LPF (low-pass filter) 202, a BPF (band-passfilter) 203, a residual error measuring section 204 and an rms noisemeasuring section 205. The LPF 202 and the BPF 203 are measuring filtersfor use to measure the residual error.

The tracking error (TE) signal that has been input to the buffer 201 isbranched into two signal components that are supplied to the LPF 202 andthe BPF 203, respectively. The residual error measuring section 204measures the residual tracking error of the tracking error signal thathas passed through the LPF 202. Meanwhile, the rms noise measuringsection 205 measures the rms noise of the tracking error signal that haspassed through the BPF 203.

FIG. 9 schematically shows the respective frequency characteristics ofthe LPF 202 and the BPF 203. The LPF 202 of the residual tracking errormeasuring section 112 has a cutoff frequency LPF_TcL and the BPF 203 ofthe residual tracking error measuring section 112 has a lower cutofffrequency BPF_TcL and a higher cutoff frequency BPF_TcH. The cutofffrequency LPF_TcL of the LPF 202 is equal to the lower cutoff frequencyBPF_TcL of the BPF 203. These cutoff frequencies may be changedaccording to the residual error measuring conditions. The LPF 202 is aButterworth filter with a gradient of −60 dB/dec, while the BPF 203 isalso a Butterworth filter with a gradient of +60 dB/dec on the lowerfrequency side and a gradient of −60 dB/dec on the higher frequencyside.

The residual error measuring section 204 detects in real time a residualtracking error included in the tracking error signal that has passedthrough the LPF 202 while the optical disc 101 is being inspected.Meanwhile, the rms noise measuring section 205 detects rms noiseincluded in the tracking error signal that has passed through the BPF203 as an effective noise component of the tracking error signal thathas been obtained in a period of time corresponding to one turn of theoptical disc.

The residual focus error measuring section 113 has the sameconfiguration as the residual tracking error measuring section 112.Specifically, the residual focus error measuring section 113 alsoincludes a buffer 301, an LPF 302, a BPF 303, a residual error measuringsection 304 and an rms noise measuring section 305 as shown in FIG. 8.The LPF 302 and the BPF 303 are measuring filters for use to measure theresidual error.

The focus error (FE) signal that has been input to the buffer 301 isbranched into two signal components that are supplied to the LPF 302 andthe BPF 303, respectively. The residual error measuring section 304measures the residual focus error of the focus error signal that haspassed through the LPF 302. Meanwhile, the rms noise measuring section305 measures the rms noise of the focus error signal that has passedthrough the BPF 303.

The LPF 302 and the BPF 303 have the same frequency characteristics asthe LPF 202 and the BPF 203, respectively. As shown in FIG. 9, the LPF302 of the residual focus error measuring section 113 has a cutofffrequency LPF_FcL and the BPF 303 of the residual focus error measuringsection 113 has a lower cutoff frequency BPF_FcL and a higher cutofffrequency BPF_FcH. The cutoff frequency LPF_FcL of the LPF 302 is equalto the lower cutoff frequency BPF_FcL of the BPF 303. These cutofffrequencies may be changed according to the residual error measuringconditions. The LPF 302 is a Butterworth filter with a gradient of −60dB/dec, while the BPF 303 is also a Butterworth filter with a gradientof +60 dB/dec on the lower frequency side and a gradient of −60 dB/decon the higher frequency side.

The residual error measuring section 304 detects in real time a residualtracking error included in the focus error signal that has passedthrough the LPF 302 while the optical disc 101 is being inspected.Meanwhile, the rms noise measuring section 305 detects rms noiseincluded in the tracking error signal that has passed through the BPF303 as an effective noise component of the focus error signal that hasbeen obtained in a period of time corresponding to one turn of theoptical disc.

Hereinafter, the cutoff frequencies of the LPFs 202, 302 and BPFs 203,303, which are used as measuring filters, and their residual errormeasuring conditions and procedures will be described.

The following Table 1 shows exemplary residual focus error measuringconditions and reference values for a 4× BD-R disc and a 6× BD-R disc.On the other hand, the following Table 2 shows exemplary residualtracking error measuring conditions and reference values for those twotypes of discs. In the following description, the 4×, 6× and otherlinear velocities will sometimes be simply referred to herein as 4×, 6×and so on.

Also, in the following description, the measuring conditions, referencevalues, and measuring method of the residual focus error and those ofthe residual tracking error will be described separately from each otherto help the reader get an idea of the present invention more easily.However, these two types of errors may be measured at the same time. Orone of the two types of errors may be measured first, and then the othertype of error may be measured. Also, the inspecting method of thispreferred embodiment may be carried out by measuring either one or bothof the residual focus and tracking errors.

TABLE 1 Type of disc 4× disc 6× disc Highest write rate (maximum 4× 4×6× recording speed) Radial range of measurement Every radial r < 36 mm r≧ 36 mm (radius) location Measuring rate (measurement 2× 2× 3× speed ofservo) Measuring filter (BPF_FcH)  20 kHz  20 kHz  30 kHz Measuringfilter (LPF_FcL and 3.2 kHz 3.2 kHz 4.8 kHz BPF_FcL) Gain crossoverfrequency of 3.2 kHz 3.2 kHz 3.2 kHz servo characteristic (crossoverfrequency) Reference value (BPF) 32 nm 32 nm 32 nm Reference value (LPF)80 nm 80 nm 110 nm 

TABLE 2 Type of disc 4× disc 6× disc Highest write rate (maximum 4× 4×6× recording speed) Radial range of measurement Every radial r < 36 mm r≧ 36 mm (radius) location Measuring rate (measurement 2× 2× 3× speed ofservo) Measuring filter (BPF_TcH)  20 kHz  20 kHz  30 kHz Measuringfilter (LPF_TcL and 3.6 kHz 3.6 kHz 3.6 kHz BPF_TcL) Gain crossoverfrequency of 3.6 kHz 3.6 kHz 3.6 kHz servo characteristic (crossoverfrequency) Reference value (BPF) 9.2 nm 9.2 nm 9.2 nm Reference value(LPF)  20 nm  20 nm  20 nm

First of all, the measuring conditions, reference values and measuringmethod of the residual focus error will be described.

In Table 1, the “highest write rate” refers to the highest possible rateof writing information on a given optical disc. In this case, a “4×disc” means a disc on which information can be written at most at 4×linear velocity that is four times as high as the standard linearvelocity (1×). That is to say, the 4× linear velocity represents thehighest write rate. On the other hand, in a 6× disc, information can bewritten at 4× linear velocity that is four times as high as the standardlinear velocity (1×) on the inner area but at 6× linear velocity on theouter area as described above. Thus, the 6× linear velocity representsthe highest write rate in this case. That is why as for the 4× disc, themeasurements are carried out under the same set of conditions over theentire area of the disc (i.e., from the innermost portion through theoutermost portion thereof). On the other hand, as for a 6× disc, themeasurements are carried out under two different sets of conditions,which are switched at a radial location r of 36 mm.

The linear velocity on the inner area will be referred to herein as a“first linear velocity Lv1”, while the linear velocity on the outer areaa “second linear velocity Lv2”. Both of the first and second linearvelocities Lv1 and Lv2 are a positive real number of times as high as astandard linear velocity of 4.917 m/sec and the second linear velocityLv2 is higher than the first linear velocity Lv1.

Information about these linear velocities that enable a read/writeoperation on an optical disc is stored in advance in a predeterminedarea of the optical disc (e.g., in a disc management area in the PICzone 1003 shown in FIG. 1).

The residual focus error is measured at a linear velocity that is a halfas high as the highest write rate. In that case, to estimate theresidual focus error to be caused when the user actually reads or writesinformation from/on a BD-R disc, the gain crossover frequency of theservo filter for use in inspection and the cutoff frequencies of themeasuring filters (i.e., LPF and BPF) are defined to be a half as highas the actual frequencies for reading and writing proportionally to thelinear velocity.

FIGS. 10A and 10B show the residual focus errors to be caused when aread/write operation is performed on the same track on the same opticaldisc at 4× linear velocity and at 2× linear velocity, respectively.

In this case, the servo filter that was used for measurements has gaincrossover frequencies of 6.4 kHz and 3.2 kHz, respectively. Also, whilethe residual errors were measured, the LPF had cutoff frequencies of 3.2kHz and 1.6 kHz. Thus, comparing these two signals, it can be seeneasily that residual error values with the same amplitude can beobtained by reducing the cutoff frequency to a half proportionally tothe ratio of the linear velocities.

Thus, in a situation where the servo characteristic is measured at alinear velocity that is a half as high as the highest write rate duringan actual write operation (i.e., a linear velocity at which user data isactually written), even if the measurements are done at two differentlinear velocities, residual error values with the same amplitude canstill be obtained by cutting down the gain crossover frequency of theservo filter and the cutoff frequencies of the measuring filters to ahalf proportionally to the ratio of the linear velocities.

If the disc were revolved at as high a rotational velocity as more than5,000 rpm, then the mechanical vibrations of the spindle motor and theresonance of the actuator of the optical pickup would raise a seriousproblem. That is to say, if the disc were revolving at such highvelocities, the influence of mechanical residual error components,produced by members of the inspecting apparatus such as a spindle motorand an actuator, would be quite a little to make it difficult toaccurately measure the target residual error components of the opticaldisc itself. However, if the residual focus error is measured with therotational velocity decreased to a half as high as the linear velocityduring an actual read/write operation and with the gain crossoverfrequency of the servo filter and the cutoff frequencies of themeasuring filters also reduced to a half proportionally to the ratio oflinear velocities, then the mechanical residual error componentsproduced by the vibrations or resonance of the inspecting apparatusitself can be reduced. As a result, the target residual error componentsof the optical disc itself can be measured accurately.

As shown in Table 1, in the 6× disc, inside of the radial location of 36mm, the highest write rate is 4×. That is why the measurements can bemade on the same measuring condition as the 4× disc. That is to say,inside of the radial location of 36 mm on the 6× disc, the residualfocus error can be measured under the same conditions as a conventional4× disc inspecting apparatus.

On the other hand, at and outside of the radial location of 36 mm, thehighest write rate is 6×. Also, the ratio of the 6× linear velocity tothe 4× linear velocity is 1.5. That is why the 6× disc could beinspected with the gain crossover frequency of the servo filter and thecutoff frequencies of the measuring filters (LPF and BPF) increased bythe factor of 1.5 compared to the values for use in doing measurementson the 4× disc. However, if the gain crossover frequency of the servofilter were multiplied by the factor of 1.5 and increased to 4.8 kHz, itwould be equivalent to a situation where an optical disc drive actuallyused by the user is performing a read/write operation at 6× linearvelocity with a focus servo control performed at a gain crossoverfrequency of 9.6 kHz.

In a small-sized optical disc drive such as a normal half-height opticaldisc drive, the gain crossover frequency needs to be 6 to 8 kHz, where acertain degree of phase margin can be ensured, in order to prevent theactuator from oscillating and to perform the servo control with goodstability. That is to say, at the half rotational frequency, a gaincrossover frequency of 3.2 kHz is virtually the limit that can beachieved by the optical disc drive. For that reason, according to thisinspecting method, in measuring the residual focus error on the outerarea of a 6× disc (i.e., at or outside of the radial location of 36 mm),the gain crossover frequency of the servo filter is defined to be 3.2kHz, which is equal to that of a 4× disc inspecting apparatus.

On the other hand, the cutoff frequencies of the measuring filters LPFand BPF are changed by the ratio of the highest linear velocity of a 6×disc to that of a 4× disc. Hereinafter, it will be described why thischange needs to be made. When it comes to a residual focus error, adecrease in the SER (symbol error rate) of an RF signal should be takeninto account. That is to say, a reference value needs to be provided forthe residual focus error in order to prevent the RF signal from losing aportion of its envelope after writing. Specifically, once the allowabledefocus margin of a disc is exceeded during a write operation whilethere is a significant residual focus error, the laser beam spot on thestorage layer of the optical disc will be widened by defocusing too muchto converge the laser beam with sufficient energy density. As a result,marks will be left with a substantial lack of recording power. That isto say, marks will be left on the storage layer of the optical disc withtheir widths varied in the radial direction according to the magnitudeof the residual error.

FIG. 11 shows what relation the read signal (RF signal) waveform and thefocus error signal will have in a situation where information is writtenon an optical disc with a significant residual focus error and then theinformation written is read. In that case, the focus error signal haspassed through the measuring filters, and therefore, the amplituderepresents the residual focus error. As can be seen from FIG. 11, wherethere is a significant residual focus error, the RF signal loses aportion of its envelope closer to the mark (which will be referred toherein as the “lower envelope”). Such a residual focus error is produceddue to a variation in the thickness of the coating layer over the discsurface. The residual focus error is measured by rotating the opticaldisc and by monitoring the level of the signal generated based on thereflected light. That is why the spatial distribution of the thicknessof the coating layer that covers the storage layer is monitored afterhaving been converted into a distribution of linear velocities of thescanning laser beam on the time axis. That is to say, the frequencycomponents of the residual focus error caused by the variation in thethickness of the coating layer are proportional to the rotational linearvelocities. For example, if the linear velocity is increased from 2× to3×, the residual focus error that is monitored at the 2× linear velocitydue to a variation in the thickness of the coating layer over the discsurface scanned with the laser beam will have its frequencies shifted toa frequency range that is 1.5 times (i.e., the ratio of these two linearvelocities) as high as the previous range. In this case, if thefrequencies of the servo filter and the measuring filters are changedproportionally to the ratio of the linear velocities, a residual focuserror with the same amplitude will be monitored as already describedwith reference to FIGS. 10A and 10B. However, since the rotationalvelocity is increased with the gain crossover frequency of the servofilter fixed at 3.2 kHz, the residual focus error component atfrequencies of 4-5 kHz in the vicinity of the gain crossover frequencycannot be suppressed at 3× rotational velocity even by performing focusservo controls because those components are outside of the gaincrossover frequency, and therefore, are monitored as more significantresidual errors. That is to say, unless the residual focus errorcomponents in this range can be suppressed, the RF signal will lose aportion of its envelope and the read signal will have a decreased SER,thus causing read errors.

For that reason, if the residual focus error is measured with the cutofffrequencies of the residual focus error measuring filters multiplied bythe factor of 1.5 proportionally to the rotational velocity, then everysingle residual focus error component that would cause the RF signal tolose a portion of its envelope on a 4× disc can be detected.

Meanwhile, the residual focus error component of the BPF is measured toreduce the amount of ineffective current flowing through the actuator,rather than ensuring good read/write signal quality. That is why such aresidual focus error component is sometimes called “rms noisecomponent”. However, if the optical disc is inspected with the lower andhigher cutoff frequencies of the band-pass filter for use to measure therms noise multiplied by the factor of 1.5 proportionally to the ratio ofthe linear velocities, every single rms noise component in a frequencyrange that would raise a problem on a 4× disc can also be detectedwithout fail.

Consequently, if the 6× disc is inspected with the cutoff frequencies ofthe measuring filters multiplied by the factor of 1.5 while the gaincrossover frequency of the servo filter fixed at 3.2 kHz, correspondingto the one to be achieved by a real drive, then discs with significantresidual focus errors can be sorted out properly.

What is more, the inspection can get done without changing thecharacteristics of the servo filter (e.g., the gain crossoverfrequencies among other things) between a 4× disc and a 6× disc orbetween the inside and outside of the switching radial location on a 6×disc. As a result, the inspecting apparatus may use the same servofilter for both purposes, which is beneficial in terms of theproductivity of optical discs, too. That is to say, a 4× disc inspectingapparatus can be used as it is as a 6× disc inspecting apparatus.

On top of that, there is no longer any need to suspend the tracking orfocus control operation, modify the settings of the reference servo, andthen resume the tracking or focus control operation and optical discinspection again in order to switch or change the servo filtersaccording to the linear velocity. Added to that, the entire disc can beinspected continuously for residual errors just by changing the linearvelocities. Consequently, the inspection can get done in a much shortertime. As a result, the tact time can be shortened and the productivityof optical discs can be increased. Furthermore, the residual errors canbe measured under the same reference servo conditions as an inspectingapparatus for 4× BD-R discs. That is to say, an inspecting apparatus for4× BD-R discs can be used as it is to inspect a 6× BD-R, too. Bycombining the respective inspection lines of these two types of opticaldiscs together in this manner, there is no need to introduce a newinspecting apparatus, thus cutting down the equipment costsignificantly. As a result, a huge number of optical discs can bemass-produced at a much lower cost, which is tremendously beneficial.

By using an inspecting apparatus for which those measuring conditionshave been defined in advance, the entire surface of the optical disc canbe inspected for residual focus errors from the innermost edge throughthe outermost edge thereof. And if the LPF and BPF residual error valuesare equal to or smaller than reference values, the disc is passed as aGO. But if the residual error values exceed the reference values, thedisc is a NO-GO.

Next, the residual focus error reference value will be described. Thereference values are preferably associated with the power margin of alaser beam while a write operation is being performed on the opticaldisc. FIG. 12 shows the relations between the residual focus errors oftwo types of discs and their defocus margins. As used herein, the“defocus margin” refers to a focus range in which SER≦4.2E−3. In thisexample, Discs A and B with mutually different power margins are used.Specifically, Discs A and B have power margins of 23% and 18%,respectively. That is to say, there is a difference of 5% in powermargin between Discs A and B.

As used herein, the “power margin” refers to a power range in which thelimit equalizer jitter falls within a predetermined range when a writeoperation is performed with the power decreased or increased from theoptimum one. More specifically, the “power margin” refers to a powerrange in which if the power has been decreased by 10%, a single-layerdisc has a jitter of 8.5% or less, L0 layer of a dual-layer disc (i.e.,the deeper layer that is more distant from the light incoming side) alsohas a jitter of 8.5% or less, and L1 layer thereof (i.e., the shallowerlayer that is closer to the light incoming side) has a jitter of 10.5%or less. Even more specifically, the power margin refers to a powerrange in which marks except the shortest mark or space have a jitter of8.5% or less on the L1 layer. For example, according to the 1-7modulation technique, the mark lengths are limited to the range of 2Tthrough 8T, and therefore, the shortest mark length is 2T. Meanwhile,the “power margin” also refers to a power range in which if the powerhas been increased by 10%, a single-layer disc has a jitter of 10.5% orless, L1 layer of a dual-layer disc also has a jitter of 10.5% or less,and L1 layer thereof has a jitter of 12.5% or less. Even morespecifically, the power margin refers to a power range in which marksexcept the shortest mark or space have a jitter of 10.5% or less on theL1 layer.

As shown in FIG. 12, even if a residual focus error of the samemagnitude has occurred on these two types of discs, the defocus marginof Disc A is always approximately 30-40 nm broader than that of Disc Bat any residual focus error value. That is to say, Disc A has a broaderpower margin than Disc B. That is why Disc A should be less affected bya decrease in recording power due to the residual focus error than DiscB is. As can be seen from the results shown in FIG. 12, if there is adifference of 5% between the power margins, then it may be determinedthat there is a tolerance of approximately 30-40 nm with respect to thedefocus margin.

That is to say, even if the residual focus error reference value weremade less strict (e.g., increased from 80 nm to 110-120 nm) according tothe power margin, both discs would still have similar system margins.For example, in a situation where the residual focus error has areference value of 80 nm, if there is a power margin of ±10%, then thereference value may be increased to 110 nm.

In other words, even if the residual focus error tolerance were extendedby the magnitude of the defocus margin with respect to such a disc witha relatively broad power margin, the margin tolerated by the overallsystem would not decrease. That is why as for a disc with a good powermargin, if the reference value is made less strict in view of theresidual focus error tolerance value, the production yield of media canbe increased without decreasing the productivity of the media with anexcessively strict residual error reference value. Also, even bydesigning such a disc with a broad power margin by either optimizing therecording film or reflective film or modifying the write strategy, forexample, the residual error tolerance, which will often pose a problemwhen the read/write rates should be increased, can also be extended andthe productivity of optical discs can be increased, which is definitelybeneficial.

Next, the residual tracking error measuring conditions, the referencevalues, and the method of inspecting the disc for residual trackingerrors will be described.

In Table 2, the “highest write rate” refers to the highest possible rateof writing information on a given optical disc as in Table 1. In thiscase, a “4× disc” means a disc on which information can be written atmost at 4× linear velocity that is four times as high as the standardlinear velocity (1×). On the other hand, in a 6× disc, information canbe written at 4× linear velocity that is four times as high as thestandard linear velocity (1×) on the inner area but at 6× linearvelocity on the outer area as described above. That is why as for the 4×disc, the measurements are carried out under the same set of conditionsover the entire area of the disc (i.e., from the innermost portionthrough the outermost portion thereof). On the other hand, as for a 6×disc, the measurements are carried out under two different sets ofconditions, which are switched at a radial location r of 36 mm. Thelinear velocity on the inner area will be referred to herein as a “firstlinear velocity Lv1”, while the linear velocity on the outer area a“second linear velocity Lv2”. Both of the first and second linearvelocities Lv1 and Lv2 are an integral number of times as high as astandard linear velocity of 4.917 m/sec and the second linear velocityLv2 is higher than the first linear velocity Lv1.

The residual tracking error is measured at a linear velocity that is ahalf as high as the highest write rate. In that case, to estimate theresidual tracking error to be caused when the user actually reads orwrites information from/on a BD-R disc, the gain crossover frequency ofthe servo filter for use in inspection and the cutoff frequencies of themeasuring filters (i.e., LPF and BPF) are defined to be a half as highas the actual frequencies for reading and writing proportionally to thelinear velocity.

In this respect, the same idea as what has already been described abouthow to determine the gain crossover frequency of the servo filter andthe cutoff frequencies of the measuring filters (LPF and BPF) in theresidual focus error measuring method is also applicable. That is tosay, in a situation where the servo characteristic is measured at alinear velocity that is a half as high as the highest write rate duringan actual write operation (i.e., a rate at which user data is actuallywritten), even if the measurements are done at two different linearvelocities, residual error values with the same amplitude can still beobtained by cutting down the gain crossover frequency of the servofilter and the cutoff frequencies of the measuring filters to a halfproportionally to the ratio of the linear velocities.

If the disc were revolved at as high a rotational velocity as more than5,000 rpm, then the mechanical vibrations of the spindle motor and theresonance of the actuator of the optical pickup would raise a seriousproblem. That is to say, if the disc were revolving at such highvelocities, the influence of mechanical residual error components,produced by members of the inspecting apparatus such as a spindle motorand an actuator, would be quite a little to make it difficult toaccurately measure the target residual error components of the opticaldisc itself. However, if the residual focus error is measured with therotational velocity decreased to a half as high as the linear velocityduring an actual read/write operation and with the gain crossoverfrequency of the servo filter and the cutoff frequencies of themeasuring filters also reduced to a half proportionally to the ratio oflinear velocities, then the mechanical residual error componentsproduced by the vibrations or resonance of the inspecting apparatusitself can be reduced. As a result, the target residual error componentsof the optical disc itself can be measured accurately.

As shown in Table 2, in the 6× disc, inside of the radial location of 36mm, the highest write rate is 4×. That is why the measurements can bemade on the same measuring condition as the 4× disc. That is to say,inside of the radial location of 36 mm on the 6× disc, the residualtracking error can be measured under the same conditions as aconventional 4× disc inspecting apparatus.

On the other hand, at and outside of the radial location of 36 mm, thehighest write rate is 6×. Also, the ratio of the 6× linear velocity tothe 4× linear velocity is 1.5. That is why the 6× disc could beinspected with the gain crossover frequency of the servo filter and thecutoff frequencies of the measuring filters (LPF and BPF) increased bythe factor of 1.5 compared to the values for use in doing measurementson the 4× disc. However, if the gain crossover frequency of the servofilter were multiplied by the factor of 1.5 and increased to 5.4 kHz, itwould be equivalent to a situation where an optical disc drive actuallyused by the user is performing a read/write operation at 6× linearvelocity with a focus servo control performed at a gain crossoverfrequency of 10.8 kHz.

In a small-sized optical disc drive such as a normal half-height opticaldisc drive, the gain crossover frequency needs to be 6 to 8 kHz, where acertain degree of phase margin can be ensured, in order to prevent theactuator from oscillating and to perform the servo control with goodstability. That is to say, at the half rotational velocity, a gaincrossover frequency of 3.6 kHz is virtually the limit that can beachieved by the optical disc drive. For that reason, according to thisinspecting method, in measuring the residual focus error on the outerarea of a 6× disc (i.e., at or outside of the radial location of 36 mm),the gain crossover frequency of the servo filter is defined to be 3.6kHz, which is equal to that of a 4× disc inspecting apparatus.

The residual tracking error is caused by a variation in the thickness ofthe optical disc in the radial direction, non-uniformity of grooves, adefect of the stamper, a scratch left during the forming process, orunevenness of the spin coated layer that forms the protective coating,for example. The residual tracking error is measured by rotating theoptical disc and by monitoring the signal generated based on thereflected light. That is why the non-uniform spatial distribution oftracks in the tracking direction is monitored after having beenconverted into a distribution of rotational linear velocities on thetime axis. That is to say, the frequency components of the residualtracking error are proportional to the rotational linear velocities. Forexample, if the rotational linear velocity is increased from 2× to 3×,the residual tracking error that is monitored at the 2× linear velocitydue to some variation in track shape in the radial direction will haveits frequencies shifted to a frequency range that is 1.5 times (i.e.,the ratio of the two linear velocities) as high as the previous range.In this case, if the frequency ranges of the servo filter and themeasuring filters are shifted toward higher frequencies proportionallyto the ratio of the linear velocities, a residual tracking error withthe same amplitude will be monitored. However, since the rotationalvelocity is increased with the gain crossover frequency of the servofilter fixed at 3.6 kHz, the residual tracking error component atfrequencies of 4-5 kHz in a higher range than the gain crossoverfrequency cannot be suppressed at 3× rotational velocity even byperforming a tracking servo controls because those components areoutside of the gain crossover frequency, and therefore, are monitored asmore significant residual tracking errors. That is to say, unless theresidual tracking error components in this range can be suppressed, thetracking error signal will have outstanding spike noise, thusthreatening the stability of tracking control.

In measuring residual focus errors on a 6× disc on the outer areathereof at and outside of the radial location of 36 mm, the cutofffrequencies of the two measuring filters including an LPF and a BPF aresupposed to be increased by the factor of 1.5 that is the ratio of thehighest velocity of the 6× disc to that of the 4× disc compared to thecutoff frequencies while the 4× disc is being inspected. This is done toprevent the SER from decreasing due to a partial loss of the envelope ofan RF signal to be caused by residual focus errors during a writeoperation.

However, a signal to be written on a BD has such a broad off-trackmargin that no significant residual tracking error will cause the RFsignal to lose any portion of its envelope or cause any decrease in SER.Rather than that, more attention should be paid to the stability oftracking servo in setting residual tracking error measuring conditions.That is to say, the disc has only to be inspected for residual trackingerror components at least in such a range in which the stability of thetracking servo is threatened.

In this case, residual tracking errors or disturbance components, ofwhich the frequencies are higher than the gain crossover frequency of3.6 kHz of the servo filter, are located outside of the tracking servocontrol range, and never have such frequencies that will affect thestability of the tracking servo control. That is why in setting thecutoff frequency of the LPF to process the tracking error signal,residual tracking errors and disturbances need to be detected just in afrequency range that is lower than the vicinity of the gain crossoverfrequency of the servo filter.

Consequently, unless the residual tracking error components that arecaused by the disc itself in such a frequency range that is lower thanthe gain crossover frequency are suppressed, the tracking servo couldsuddenly fail due to some disturbance during a read/write operation. Forexample, an unintentional track jump could occur during a writeoperation to write data on a neighboring track accidentally and erasethe stored data by mistake. Meanwhile, if optical discs were producedwith high mechanical precision by going so far as to shift the frequencyrange of the measuring filter toward higher frequencies meaninglesslyand detect unnecessary residual tracking error components for nothing,the production yield of optical discs would decrease significantly. Thatis why to avoid such an undue increase in manufacturing cost, it isimportant to do inspection with an appropriate cutoff frequency definedfor the LPF.

For that reason, when the residual tracking errors of a 6× disc aremeasured on the outer area thereof at and outside of the radial locationof 36 mm, the LPF that is one of the residual tracking error measuringfilters preferably has a cutoff frequency of 3.6 kHz, which is as highas the gain crossover frequency. That is to say, as far as the cutofffrequency of the LPF is concerned, even when the residual focus errorsof a 6× disc are measured on the outer area thereof at and outside ofthe radial location of 36 mm, the same condition as the one adopted by a4× disc inspecting apparatus is preferably used. Then, every singleresidual tracking error component can be detected to ensure the servostability. As a result, it is possible to avoid any decrease in theyield of media without threatening the servo stability.

Meanwhile, as for the residual tracking error component of the BPF, astandard is set to reduce the amount of ineffective current flowingthrough the actuator, rather than ensuring servo stability. That is whysuch a residual focus error component is sometimes called “rms noise”.However, if that rms noise is also measured with the lower and highercutoff frequencies of the BPF for use to measure the rms noisemultiplied by the factor of 1.5 proportionally to the ratio of linearvelocities, every single rms noise component in a frequency range thatwould cause a problem on a 4× disc can also be detected without fail.

FIG. 13 shows how the probability of tracking failures changes with theresidual tracking error according to the frequency of disturbance. Theresults shown in FIG. 13 were obtained in the following manner.

Specifically, with the applied voltages changed into several differentvalues, the magnitudes of residual tracking errors were measured in thetracking ON state at various disturbance frequencies. Next, at each ofthe disturbance frequencies and each of the applied voltages at whichthe magnitudes of the tracking errors were measured, attempts were madein the tracking OFF state to establish a tracking servo loop (i.e., toaccomplish the tracking ON state). These attempts were made a number oftimes, and it was counted how many times the tracking servo loop couldnot be established and tracking failures occurred and how many times thetracking ON state was accomplished and could maintain good stability.Then, the ratios of the number of times of failures to the overallnumber of times of attempts were obtained to draw up a table. Themeasurements were done under such conditions that the disc had arotational velocity of 3× and the servo filter had a gain crossoverfrequency of 3.6 kHz.

In FIG. 13, the polygons 1101, 1102, 1103 and 1104 represent situationswhere the frequencies of the disturbances provoked were 1.2 kHz, 1.8kHz, 3.6 kHz, and 5.4 kHz, respectively. If the disturbances provokedhad lower frequencies than the gain crossover frequencies of the servofilters as represented by the curves 1101 and 1102 and if the residualtracking error exceeded 25 nm, the probability of tracking failuresincreased steeply. On the other hand, in a situation where thedisturbances provoked had frequencies equal to or higher than the gaincrossover frequencies of the servo filters as represented by the curves1103 and 1104, even if the residual tracking error exceeded 25 nm, theprobability of tracking failures did not increase significantly. That isto say, the present inventors confirmed that the residual tracking errorcomponents above the gain crossover frequency of the servo filter didnot affect the stability of the tracking servo. The present inventorsalso confirmed that even in a situation where the residual trackingerror components had frequencies equal to or lower than the gaincrossover frequency, if the residual tracking error components wereequal to or smaller than 25 nm, the stability was not affected whileattempts were being made to establish the tracking servo loop.

For that reason, when the residual tracking errors of a 6× disc aremeasured on the outer area thereof at and outside of the radial locationof 36 mm, the LPF that is one of the residual tracking error measuringfilters preferably has a cutoff frequency of at least 3.6 kHz, which isas high as the gain crossover frequency. That is to say, as far as thecutoff frequency of the LPF is concerned, even when the residual focuserrors of a 6× disc are measured on the outer area thereof at andoutside of the radial location of 0.36 mm, the same condition as the oneadopted by a 4× disc inspecting apparatus is preferably used. Or thecutoff frequency of the LPF may also be higher than the one adopted bythe 4× disc inspecting apparatus.

As described above, if the 6× disc is inspected for residual trackingerrors, the cutoff frequencies of the measuring filters are equalizedwith the gain crossover frequency while the gain crossover frequency ofthe servo filter maintained at 3.6 kHz, corresponding to the gaincrossover frequency to be achieved by a real drive, in order to increasethe servo stability and the production yield of optical discs. But theother measuring conditions may be the same as the ones shown in Table 2.Then, discs that would cause significant residual tracking errors can besingled out just as intended. By removing such optical discs, it ispossible to avoid an unwanted situation where the tracking servosuddenly fails due to some disturbance or a situation where asignificant residual tracking error causes an unintentional track jumpduring a write operation to write data on a neighboring trackaccidentally and/or erase the stored data by mistake.

What is more, the inspection can get done without changing thecharacteristics of the servo filter (e.g., the gain crossoverfrequencies among other things) between a 4× disc and a 6× disc orbetween the inside and outside of the switching radial location on a 6×disc. As a result, the inspecting apparatus may use the same servofilter for both purposes, which is beneficial in terms of theproductivity of optical discs, too. That is to say, a 4× disc inspectingapparatus can be used as it is as a 6× disc inspecting apparatus. Bycombining the respective inspection lines together in this manner, thereis no need to introduce a new inspecting apparatus, thus cutting downthe equipment cost significantly. As a result, a huge number of opticaldiscs can be mass-produced at a much lower cost, which is tremendouslybeneficial.

On top of that, there is no longer any need to suspend the tracking orfocus control operation, modify the settings of the reference servo andthen resume the tracking or focus control operation and optical discinspection operation again in order to switch or change the servofilters according to the linear velocity. Thus, the entire surface ofthe disc can be inspected continuously for residual errors betweendifferent linear velocities just by changing the linear velocities.Consequently, the inspection can get done in a much shorter time. As aresult, the tact time can be shortened and the productivity of theoptical discs can be increased, thus cutting down the costs.

By using an inspecting apparatus for which those measuring conditionshave been defined in advance, the entire surface of the optical disc canbe inspected for residual tracking errors from the innermost edgethrough the outermost edge thereof. And if the residual tracking errorvalues of the signals that have passed through the LPF and BPF are equalto or smaller than the reference values, the disc is passed as a GO. Butif the residual error values exceed the reference values, the disc is aNO-GO.

The optical disc inspecting method of this preferred embodiment ispreferably carried out by executing a program that instructs theinspecting apparatus to follow the inspection procedure described above.Such a program may be executed either by using a dedicated LSI built inthe inspecting apparatus or by getting the data processing done by anexternal PC. Still alternatively, the program may also be executed witha piece of dedicated hardware.

Also, in measuring the residual errors, the cutoff frequencies of theLPF and BPF are switched outside of the control loop of the opticaldisc. That is why even if the cutoff frequencies of the LPF or the BPFare changed while a focus control or a tracking control is beingperformed, naturally the servo operation is not affected at all.Consequently, the amount of time it takes to get the inspection donenever increases due to the change of filters.

Next, it will be described how to handle an optical disc that has turnedout to have a residual focus or tracking error that exceeds thereference value as a result of the inspection.

Specifically, suppose a 6× disc has turned out to have a residual focuserror and a residual tracking error, at least one of which exceedsreference values, when inspected by the optical disc inspecting methodof this preferred embodiment.

In that case, the memory 114 is searched as shown in FIG. 5 for a pieceof information about the innermost radial location at which the residualfocus or tracking error exceeds the reference value among pieces ofinformation about a single or multiple radial locations where theresidual focus or tracking error exceeds the reference value.

Such a piece of information about the innermost radial location wherethe reference value is exceeded will be identified herein by Rx. If Rxis located inside of the switching radial location, then such an opticaldisc is determined to be a NO-GO.

However, if Rx is located outside of the switching radial location, thenan area of that disc outside of the Rx location is inspected again underthe same conditions as the ones for a 4× disc. And if the disc turns outto be a GO under the inspection conditions for a 4× disc, then such anoptical disc satisfies the 4× disc residual error conditions overall.That is why such an optical disc may be used as a 4× disc, instead of a6× disc. Hereinafter, it will be described how to use such an opticaldisc as a 4× disc.

First of all, since such an optical disc was originally intended as a 6×disc, the information stored in its disc management area (PIC zone)already includes conditions for performing a write operation on it at1×, 2×, 4× and 6× linear velocities (such as power and write strategyinformation for the highest linear velocity and every other linearvelocity). In that case, the optical disc drive normally recognizes itas a 6× disc and may perform a write operation on it at the 6× linearvelocity at most. The information stored in the PIC zone is notalterable because that zone is a read-only area.

For that reason, an additional area on which information for regulatingor specifying the upper limit of the highest linear velocity can bewritten after the inspection is preferably provided for the 6× disc bymodifying the physical format. And such a piece of information about thehighest linear velocity Sx is written on that additional area as aresult of the inspection. In that case, the optical disc drive isdesigned such that the information about the highest linear velocitythat has been written on the additional area is given a higher prioritythan the information about the highest linear velocity that is stored inadvance in the disc management area.

First, the optical disc drive determines whether or not the highestlinear velocity information Sx has been written yet. If the answer isYES, the optical disc drive performs a write operation on a givenoptical disc following the Sx value (i.e., with Sx regarded as thehighest linear velocity).

The information Sx may be written on the BCA that is a disc managementarea of the optical disc, the lead-in zone, the lead-out zone, or anyother zone where the information may be added. For example, the highestlinear velocity information Sx may be written on a PAC, a DMA or an OPCtest zone inside at least one of the lead-in and lead-out zones and/orat least one of a Drive area and a Drive Calibration Zone (DCZ). Asdescribed above, the highest linear velocity information is preferablywritten on a dedicated area secured by the physical format standard. Inthat case, the upper limit of the write rate can be determined based onthe result of the inspection and according to the quality of themechanical property of the given optical disc. As a result, theproduction yield of optical discs can be increased significantly, thusincreasing the productivity and cutting down the cost by leaps andbounds.

Next, if the disc that was made as a 6× disc has turned out, as a resultof the inspection described above, to be usable as a 4× disc, then theradial location information Rx obtained by the inspection describedabove, as well as the Sx information, is written on a management area ofthe optical disc. Hereinafter, it will be described how to write thatpiece of information where.

As described above, the information stored in the PIC zone in the discmanagement area is not alterable because that zone is a read-only area.For that reason, an additional area on which information for regulatingor specifying the upper limit of the radial location where the writeoperation can be performed at the highest linear velocity after theinspection is preferably provided for the 6× disc by modifying thephysical format. And on that additional area, written is a piece ofinformation Rx about the innermost radial location at which the residualfocus or tracking error exceeds the reference value among pieces ofinformation about a single or multiple radial locations where theresidual focus or tracking error exceeds the reference value. In placeof, or in addition to, the radial location information, physical addressinformation (physical ADPI address, PAA) may also be written there. As aresult, the write operation can be performed at the highest linearvelocity 6× at inner radial locations inside of Rx but may be performedat a decreased highest linear velocity 4× at radial locations outside ofRx where the residual errors increase. As a result, the write operationcan be performed as quickly as possible according to the mechanicalprecision of the optical disc (i.e., can get done in a shorter time).

First, the optical disc drive determines whether or not the highestlinear velocity information Sx has been written yet. If the answer isYES, the optical disc drive performs a write operation on the outer areaof a given optical disc (i.e., outside of the switching radial locationof 36 mm) at most at the highest linear velocity in accordance with thehighest linear velocity information. However, if Rx has been written aswell, then the write operation is performed at the highest linearvelocity 6× at radial locations between the switching radial location of36 mm and Rx radial location but at the 4× linear velocity at radiallocations outside of Rx.

Rx may be written on the BCA that is a disc management area of theoptical disc, the lead-in zone, the lead-out zone, or any other zonewhere the information may be added. For example, the radial locationinformation Rx may be written on a PAC, a DMA or an OPC test zone insideat least one of the lead-in and lead-out zones and/or at least one of aDrive area and a Drive Calibration Zone (DCZ). Optionally, the physicaladdress information may be written instead of, or in addition to, theradial location information Rx. A dedicated area for storing the radiallocation information is preferably secured by the physical formatstandard. In that case, the upper limit of the radial location can bedetermined based on the result of the test and according to the qualityof the mechanical property of the given optical disc. As a result, thewrite operation can be performed as quickly as possible according to themechanical precision of the given disc (i.e., can get done in a shortertime), which is beneficial for the user.

On the optical disc, either both or one of the highest linear velocityinformation (Sx) and the radial location information (Rx) may bewritten. If these two pieces of information Sx and Rx are used incombination, the write operation needs to be performed with the linearvelocities changed as in the following Table 3:

TABLE 3 Write ranges and Sx Rx (mm) linear velocities 6x 57 24-36 mm: 4x36-57 mm: 6x 57-58 mm: 4x 6x Non specified 24-36 mm: 4x 36-58 mm: 6x 4x57 24-57 mm: 4x 57-58 mm: 2x 4x Non specified 24-58 mm: 4x Non specified57 24-36 mm: 4x 36-57 mm: 6x 57-58 mm: 4x Non specified Non specified24-36 mm: 4x 36-58 mm: 6x

As can be seen from this Table 3,

-   -   If Sx=6× and Rx=57 mm, then the write operation needs to be        performed at 4× linear velocity at radial locations of 24 mm to        36 mm, at 6× linear velocity at radial locations of 36 mm to 57        mm, and at 4× linear velocity at radial locations of 57 mm        through 58 mm;    -   If Sx=6× but no Rx is specified, then the write operation needs        to be performed at 4× linear velocity at radial locations of 24        mm to 36 mm and at 6× linear velocity at radial locations of 36        mm through 58 mm;    -   If Sx=4× and Rx=57 mm, then the write operation needs to be        performed at 4× linear velocity at radial locations of 24 mm to        57 mm and at 2× linear velocity at radial locations of 57 mm        through 58 mm;    -   If Sx=4× but no Rx is specified, then the write operation needs        to be performed at 4× linear velocity at every radial location        of 24 mm through 58 mm;    -   If Sx is not specified but Rx=57 mm, then the write operation        needs to be performed at 4× linear velocity at radial locations        of 24 mm to 36 mm, at 6× linear velocity at radial locations of        36 mm to 57 mm and at 4× linear velocity at radial locations of        57 mm to 58 mm; and    -   If neither Sx nor Rx is specified, then the write operation        needs to be performed at 4× linear velocity at radial locations        of 24 mm to 36 mm and at 6× linear velocity at radial locations        of 36 mm through 58 mm.

Unless Sx is specified, the highest write rate stored in the PIC zone isgiven a higher priority. On the other hand, if Rx is not specified, thenthe write operation may be performed either at 6× linear velocityoutside of the switching radial location or at 4× linear velocity overthe entire surface of the optical disc.

By writing Sx and/or Rx as additional pieces of information as describedabove, the production yield of optical discs can be increased, thusincreasing the productivity and cutting down the cost at the same time.On top of that, the upper limit of the radial location can be determinedbased on the result of the inspection and according to the quality ofthe mechanical property of the given optical disc. As a result, thewrite operation can be performed as quickly as possible according to themechanical precision of the given disc (i.e., can get done in a shortertime).

In the preferred embodiment described above, information Rx about theinnermost radial location, which is one of numerous pieces ofinformation about a single or multiple radial locations where either theresidual focus error or residual tracking error exceeds its referencevalue, is written. And at and outside of that radial location, it isdetermined, on the inspection conditions for a 4× disc, whether or notthe given disc is a GO or a NO-GO, thereby using that disc as a 4× discat and outside of that radial location. However, if there is any areathat can pass the test on 6× disc conditions at and outside of thatradial location, then the radial location information and/or a physicaladdress indicating that area may be written as the optical disc's properinformation R′X on the BCA that is a disc management area, the lead-inzone, the lead-out zone, or any other appropriate write-once area. Inthat case, if the optical disc drive consults those pieces ofinformation, a read/write operation may be performed at 6× linearvelocity at the location indicated by R′X even outside of Rx, and may beperformed at 4× linear velocity at the other locations outside of Rx. Asa result, a write operation can get done in a shorter time.

Furthermore, the Rx and Sx information may also be written even if thegiven disc has passed the test as a 6× disc. Alternatively, even if thegiven disc has passed the test as a 6× disc, that disc may also be usedas a 4× disc with Sx regarded to be 4× on purpose. In that case, thereis no need to manufacture those two types of optical discs (i.e., 6×discs and 4× discs) with the equipment changed but either 6× discs or 4×discs may be just manufactured so as to strike an adequate balancebetween demand and supply without changing the manufacturing facilities.Added to that, the same stamper may be used for both 4× discs and 6×discs, and therefore, there is no need to make multiple stampers. As aresult, the equipment cost, and eventually the overall manufacturingcost, can be cut down significantly.

Hereinafter, it will be described how to handle a disc that actually isa 4× disc but has turned out to satisfy the residual focus and trackingerror requirements for a 6× disc as a result of the inspection.

If a 4× disc has been inspected to see if the disc satisfies theresidual focus and tracking error requirements for 4× and 6× discs andif the 4× has turned out to have a mechanical precision that iscomparable to that of a 6× disc, then the 4× disc has mechanicalprecision that is high enough to use it as a 6× disc. That is to say, interms of performance, the 4× disc could be used as a 6× disc. However,in the PIC zone within the disc management area of the 4× disc, storedin advance are conditions for performing a write operation on it at 1×,2×, 4× and 6× linear velocities (e.g., power and write strategyinformation at the highest linear velocity an at every other linearvelocity). In that case, the optical disc drive normally recognizes itas a 4× disc and may perform a write operation on it at the 4× linearvelocity at most.

The information stored in the PIC zone is not alterable because thatzone is a read-only area. That is why an additional storage area tostore the highest linear velocity information (Sx) after the inspectionis provided for the 4× disc by modifying the physical format. And basedon a result of inspection, information about the highest linear velocity(6×) is stored in that additional storage area.

The optical disc drive determines whether or not the highest linearvelocity information Sx has been written on the 4× disc yet. If theanswer is YES, the optical disc drive performs a write operation on agiven optical disc in accordance with the highest linear velocityinformation (i.e., at 6× in this example).

The information Sx about the highest linear velocity may be written onthe BCA that is a disc management area of the optical disc, the lead-inzone, the lead-out zone, or any other zone where the information may beadded. For example, the highest linear velocity information Sx may bewritten on a PAC, a DMA or an OPC test zone inside at least one of thelead-in and lead-out zones and/or at least one of a Drive area and aDrive Calibration Zone (DCZ). As a result, the upper limit of the writerate can be determined based on the result of the inspection andaccording to the quality of the mechanical property of the given medium.Consequently, the production yield of optical discs can be increasedsignificantly, thus increasing the productivity and cutting down thecost by leaps and bounds. As described above, if a 4× disc has turnedout to be as good as a 6× disc as a result of the residual error test,then an overdrive write operation may be performed on it with the 4×disc supposed to be a 6× disc. Compared to a normal 4× optical disc withno residual error test result stored in its management information area,the optical disc drive can perform an overdrive write operation on sucha pseudo-6× disc with more reliability and more quickly.

By writing Rx and Sx as described above, various effects are achieved.Firstly, as far as productivity is concerned, 4× discs and 6× discs canbe inspected by a single line and only discs with good enough propertiescan be sorted out as 6× discs. Also, if only 6× discs were manufacturedand if the production yield turned out to be low as a result of theresidual error test, then quite a few of those discs should be thrownaway. However, just by writing those pieces of additional information Rxand Sx, such a risk can be reduced significantly, the productivity ofthe discs can be increased, and the overall cost can be cut down. Also,generally speaking, in a situation where discs are formed using the samestamper continuously, the greater the number of discs manufactured, theworse their mechanical property and the lower the production yield tendto be. Even so, those discs with deteriorated mechanical properties canstill be used as 4× discs because the requirements about residual errorsfor 4× discs are less strict than 6× discs'. That is why discs aremanufactured only as 6× discs during an early stage of the manufacturingprocess when the stamper is still new. But even when the stamper hasdeteriorated to the point that some of those discs turn out to fallshort of the 6× disc residual error standard but do meet the 4× discresidual error standard, such discs may continue to be manufactured as4× discs. As a result, the number of discs that can be manufactured withthe same stamper can be increased without decreasing the productionyield. That is to say, the life of the stamper can be extended andeventually the overall manufacturing cost can be cut down.

In the example described above, Sx and Rx are supposed to be written asadditional information and the discs are used as either 4× discs or 6×discs based on the result of the residual error test. In that case, thediscs may naturally be inspected separately in advance in terms ofmechanical properties, read/write performances and other properties, notjust residual errors.

Also, when the optical disc is inspected for the residual focus andtracking errors, the laser beam irradiating the optical disc is supposedto maintain constant readout power, no matter whether the linearvelocities have been switched or not. By inspecting the optical discwith the same readout power before and after the linear velocities areswitched, there is no need to adjust the circuit offset of theinspecting apparatus, which should be done if the readout powers werechanged, thus getting the inspection done in a shorter time.

Furthermore, if the disc is supposed to be rotated at two differentlinear velocities with the readout power of the radiation kept constantfor the two velocities, more damage will be done by the readoutradiation on the optical disc when it is rotated at the lower one of thetwo linear velocities. That is why while rotating at the lower linearvelocity, the optical disc may be subjected to a read durability test.By testing the disc with the same readout power in this manner, the discdoes not have to be subjected to a read durability test at multipledifferent linear velocities, thus contributing to increasing theproductivity of optical discs.

The inspecting method of the present invention is applicable to both HTL(high to low) and LTH (low to high) types of BD-Rs. Also, the inspectingmethod of the present invention is applicable to both single-layeroptical discs and dual-layer optical discs alike.

Furthermore, in the foregoing description, the inspecting method of thepresent invention is supposed to be applied to a BD-R. However, the samemethod is applicable to a rewritable BD-RE and a read-only BD-ROM, too.

In the foregoing description, the optical disc inspecting method of thepresent invention is supposed to be used to inspect 6× discs. However,the same method can be naturally used to inspect 8× discs, or opticaldiscs on which information is supposed to be written at even higherrates, for residual errors.

An 8× disc may have two radial locations to switch the linear velocitiesat as shown in FIG. 4. That is why on an 8× disc, a CLV write operationcan be performed with the linear velocities changed in three stagesaccording to the radial location, e.g., at 4× linear velocity inside ofthe inner radial location of 36 mm, at 6× linear velocity between thetwo switching radial locations of 36 mm and 48 mm, and then at 8× linearvelocity at and outside of the outer radial location of 48 mm. FIG. 4shows the relations between the radial location and rotational velocityin a situation where a CLV read/write operation is performed on an 8×disc with the linear velocities changed according to the radial locationfrom 4× into 6× and then into 8×. The rotational velocity at thereference radial location in the innermost part of the 4× CLV area isdefined to be the highest possible rotational velocity for therespective linear velocities. Specifically, the first switching radiallocation may be defined somewhere between approximately 33-36 mm and thesecond switching radial location may be defined somewhere betweenapproximately 44-48 mm. And the write operation may be performed at 4×linear velocity inside of the first switching radial location, at 6×linear velocity between the first and second switching radial locations,and then at 8× linear velocity outside of the second switching radiallocation. In that case, compared to the example described above in whichthe linear velocities are switched between 4× and 8× at only one radiallocation, the overall transfer rate can be increased and the write timecan be shortened because the write operation can be carried out at 6×,not 4×, between the first and second switching radial locations. Andeven such an 8× disc can also be inspected for residual focus andtracking errors by the same method as what has already been described.

Alternatively, in inspecting a 6× disc, the linear velocities may alsobe changed between 4× and 6× as shown in FIG. 14. Specifically, in thatcase, a constant angular velocity (CAV) write operation may be performedat a rotational velocity corresponding to 4× (i.e., approximately 8,000rpm if r=24 mm) in the innermost area inside of the first switchingradial location, and the modes of the write operation may be changedinto 6× CLV at a switching radial location of approximately 33-36 mm. Inthat case, the highest write rate is achieved and data can be written ona single disc in the shortest time.

In a situation where the area on which the write operation is supposedto be performed by the CAV control technique is inspected for residualerrors, the residual error test is started at a rotational velocity thatis a half as high as the CAV rotational velocity and then the linearvelocity gradually increases as the test beam goes farther toward outerradial locations. That is why until the switching radial location isreached, the residual error test may be carried out with the cutofffrequencies of the measuring filters (which are an LPF and a BPF ininspecting the disc for residual focus errors and a BPF in inspectingthe disc for residual tracking errors) changed according to the radiallocation. And when and after the switching radial location is reached,the residual error test may be carried out by the inspecting methoddescribed above.

In the preferred embodiments described above, to determine a switchingradial location, a radial location, where the linear velocity becomesthe highest rotational velocity while a read/write operation isperformed inside of the switching radial location, is found. On a 6×disc, the switching radial location is within the range of approximately33 mm to 36 mm as described above. That is why the switching radiallocation may also be determined within this range even without findingthe radial location where the rotational velocity will be the highest.Also, no matter whether or not such a radial location where therotational velocity will be the highest has been found to determine theswitching radial location, the residual error does not have to bemeasured at that radial location where the rotational velocity will bethe highest. This is because the radial location where the linearvelocity during a read/write operation will be the highest rotationalvelocity inside of the switching radial location may not belong to theuser data area. That is why the 6× disc may be inspected for residualerrors under the same conditions as the ones for a 4× disc inside of theswitching radial location and under the same conditions as what hasalready been described at and outside of the switching radial location.

Also, in the preferred embodiments described above, the switching radiallocation (i.e., a reference radial location where the measuringvelocities are switched) for a 6× disc (i.e., a disc on which aread/write operation can be performed at 6× linear velocity at least atsome radial location) is defined at 36 mm. And the residual errors aresupposed to be measured at 2× linear velocity as in a 4× disc inside ofthe radial location of 36 mm and at 3× linear velocity at and outside ofthe radial location of 36 mm. However, since the “switching radiallocation” is a boundary at which the linear velocities are switched, theresidual error may be measured at that switching radial location ateither 2× linear velocity or 3× linear velocity. That is to say, theresidual errors may be measured at 2× linear velocity as in a 4× disc atand inside of the radial location of 36 mm and at 3× linear velocityoutside of the radial location of 36 mm.

As described above, the present invention provides an optical disc inwhich the rotational velocities are switched between the first andsecond linear velocities Lv1 and Lv2 (where Lv1<Lv2) at a radiallocation where one of the two linear velocities reaches the same highestrotational velocity as the other linear velocity's. The presentinvention also provides a write-once (or rewritable) optical disc suchas a BD on which a write operation can be performed at as high a linearvelocity as 6× or at even higher velocities by adopting an optical discresidual error inspecting method. According to that method, the opticaldisc is inspected for residual errors at a rotational velocity that is ahalf as high as the write rate on the disc. The residual error(residual) is measured based on a focus error signal and a trackingerror signal under such measuring conditions on which the two rotationalvelocities have the same servo filter characteristic (i.e., the gaincrossover frequency). And it is determined whether or not the residualerror falls within a prescribed range. The present invention furtherprovides a method and apparatus for inspecting the optical discprecisely such that good write signal quality and servo stability areachieved when a signal is written on such an optical disc. And thepresent invention further provides a method for writing a signal ofquality on such an optical disc.

The present invention has been described in detail by way of specificpreferred embodiments. However, the present invention may also bedefined as follows. Specifically, an optical information storage mediuminspecting method according to the present invention is a method forinspecting an optical information storage medium for residual errors ofa focus error signal or a tracking error signal. The method ischaracterized by including the steps of: irradiating the opticalinformation storage medium with a laser beam and rotating the opticalinformation storage medium by a constant linear velocity (CLV) controltechnique by reference to the radial location at which the laser beamforms a spot on the storage medium and changing the rotationalvelocities according to the radial location on the storage mediumbetween at least two linear velocities that include a first linearvelocity Lv1 and a second linear velocity Lv2, where Lv1<Lv2; performinga focus control and a tracking control on the optical informationstorage medium to generate a focus error signal and a tracking errorsignal based on the light that has been reflected from the opticalinformation storage medium; getting the focus error signal and thetracking error signal processed by their associated types of frequencyband-elimination filters to obtain respective residual errors of thefocus and tracking error signals; and comparing the residual errors topredetermined reference values, thereby determining whether or not theresidual errors fall within prescribed ranges of the reference values.

In one preferred embodiment, the optical information storage medium isinspected by being rotated at the first linear velocity Lv1 in an areabetween first and second radial locations R1 and R2 on the storagemedium, where R1<R2. On the other hand, the optical information storagemedium is inspected by being rotated at the second linear velocity Lv2at or outside of the second radial location R2 on the storage medium.

In another preferred embodiment, the Lv2/Lv1 ratio of the second linearvelocity Lv2 to the first linear velocity Lv1 is either 1.5 or 2.

In still another preferred embodiment, the first linear velocity Lv1 is9.834 m/sec.

In yet another preferred embodiment, if Lv2/Lv1=1.5, the second radiallocation R2 satisfies 33 mm≦R2≦36 mm but if Lv2/Lv1=2.0, the secondradial location R2 satisfies 44 mm≦R2≦48 mm.

In yet another preferred embodiment, the two radial locations R1 and R2and the two linear velocities Lv1 and Lv2 satisfy R2/R1=Lv2/Lv1.

In yet another preferred embodiment, the highest rotational velocity ofthe medium being inspected at the first linear velocity is approximatelyequal to that of the medium being inspected at the second linearvelocity.

In yet another preferred embodiment, the inspection is done at the firstor second linear velocity that is a half or less as high as a linearvelocity for writing that is stored in advance in a disc management area(PIC zone) on the optical information storage medium.

In yet another preferred embodiment, the inspection is done such that nomatter whether the storage medium is being rotated at the first linearvelocity or the second linear velocity, the servo characteristic of thefocus control maintains the same gain crossover frequency and the servocharacteristic of the tracking control also maintains the same gaincrossover frequency.

In yet another preferred embodiment, the focus error signal is suppliedto both of two different types of filters included in the frequencyband-elimination filter for the focus error signal. The two filters area low-pass filter (LPF) with a cutoff frequency FcL and a band-passfilter (BPF), of which the lower and higher cutoff frequencies are FcLand FcH, respectively. The frequencies FcL and FcH are changed on ascalable basis according to the ratio of the second linear velocity tothe first linear velocity.

In yet another preferred embodiment, the tracking error signal issupplied to both of two different types of filters included in thefrequency band-elimination filter for the tracking error signal. The twofilters are a low-pass filter (LPF) with a cutoff frequency TcL and aband-pass filter (BPF), of which the lower and higher cutoff frequenciesare TcL and TcH, respectively. TcL is constant irrespective of the ratioof the second linear velocity to the first linear velocity and TcH ischanged on a scalable basis according to the ratio of the second linearvelocity to the first linear velocity.

In yet another preferred embodiment, the values F_LPF and F_BPF in twodifferent frequency bands of the focus error signal that has passedthrough the LPF and the BPF, respectively, are compared to theirpredetermined reference value, thereby determining whether or not theF_LPF and F_BPF values fall within their associated prescribed range.The values T_LPF and T_BPF in two different frequency bands of thetracking error signal that has passed through the LPF and the BPF,respectively, are compared to their predetermined reference value,thereby determining whether or not the T_LPF and T_BPF values fallwithin their associated prescribed range.

In yet another preferred embodiment, if the F_LPF, F_BPF, T_LPF andT_BPF values are compared to the predetermined reference values at oneradial location after another, then these four values are compared totwo different sets of predetermined reference values that are associatedwith the first and second linear velocities, respectively.

In yet another preferred embodiment, the reference value for the F_LPFvalue at the second linear velocity is approximately equal to or greaterthan the reference value for the F_LPF value at the first linearvelocity.

In yet another preferred embodiment, when the storage medium isinspected for residual focus and tracking errors, the readout power ofthe laser beam to irradiate the optical information storage medium iskept constant irrespective of the linear velocity.

An inspecting apparatus according to the present invention is anapparatus for inspecting an optical information storage medium for theresidual errors of a focus error signal and a tracking error signal. Theapparatus includes: an optical pickup for irradiating the opticalinformation storage medium with a laser beam; a spindle motor forrotating the optical information storage medium; a rotational velocitysetting section for performing a constant linear velocity (CLV) controlby reference to the radial location at which the laser beam forms a spoton the storage medium and changing the rotational velocities accordingto the radial location on the storage medium between at least two linearvelocities that include a first linear velocity Lv1 and a second linearvelocity Lv2, where Lv1<Lv2; a focus signal residual error measuringsection for performing a focus control to generate a focus error signalbased on the light reflected from the optical information storage mediumand measuring the residual error (residual) of the focus error signal bythe level of the focus error signal; a tracking signal residual errormeasuring section for performing a tracking control to generate atracking error signal based on the light reflected from the opticalinformation storage medium and measuring the residual error (residual)of the tracking error signal by the level of the tracking error signal;a memory for retaining the residual errors that have been measured ateach radial location by the focus signal residual error measuringsection and the tracking signal residual error measuring section; and adecision section for comparing the residual errors measured topredetermined residual focus and tracking error reference values,thereby determining whether or not the residual errors fall withinprescribed ranges of the reference values.

In one preferred embodiment, the residual error (residual) of the focuserror signal is obtained by getting the focus error signal processed byits associated type of frequency band-elimination filter, which isprovided in the focus signal residual error measuring section. Theresidual error (residual) of the tracking error signal is obtained bygetting the tracking error signal processed by its associated type offrequency band-elimination filter, which is provided in the trackingsignal residual error measuring section.

In another preferred embodiment, information is written on an opticalinformation storage medium by irradiating the storage medium with alaser beam and rotating the medium by a constant linear velocity (CLV)control technique by reference to the radial location at which the laserbeam forms a spot on the storage medium. Data is written by rotating themedium at a third linear velocity Lv3 within an area of the mediumbetween first and second radial locations R1 and R2, where R1<R2. On theother hand, data is written by rotating the medium at a fourth linearvelocity Lv4 (where Lv3<Lv4) at or outside of the second radial locationR2 on the storage medium. And the radial location R2 to change thelinear velocities is determined such that the highest rotationalvelocity of the third linear velocity at R1 becomes approximately equalto that of the fourth linear velocity at R2.

In one preferred embodiment, the third linear velocity Lv3 is 19.7m/sec, the fourth linear velocity Lv4 is either 29.5 m/sec or 39.4m/sec, and the second radial location R2 satisfies 33 mm≦R2≦36 mm ifLv4/Lv3=1.5 and satisfies 44 mm≦R2≦48 mm if Lv4/Lv3=2.0.

According to a method of writing information on an optical informationstorage medium according to the present invention, the residual focusand tracking errors are obtained by getting the memory searched by theapparatus described above. If the residual focus and tracking errorsexceed their prescribed ranges of the reference values, informationabout the innermost radial location Rx is searched for among multiplepieces of information about radial locations where one or multipleprescribed ranges are exceeded. And the information about the innermostradial location Rx is written on a BCA of the optical informationstorage medium and/or on a predetermined area in at least one of lead-inand lead-out zones to which an additional piece of information can bewritten. Optionally, instead of, or in addition to, the radial locationinformation, physical address information may also be written there.

According to a method of writing information on an optical informationstorage medium according to the present invention, the residual focusand tracking errors are obtained by getting the memory searched by theapparatus described above, thereby determining whether or not theresidual focus or tracking error satisfies the prescribed range of itsassociated reference values of linear velocities. And based on theresult of the decision, the highest writable linear velocity (Sx) iswritten on a BCA of the optical information storage medium and/or on apredetermined area in at least one of lead-in and lead-out zones towhich an additional piece of information can be written.

In one preferred embodiment, the highest writable linear velocity Sxand/or the radial location information Rx is/are written on a PAC, a DMAor an OPC test zone inside at least one of the lead-in and lead-outzones and/or at least one of a Drive area and a Drive Calibration Zone(DCZ). Optionally, instead of, or in addition to, the radial locationinformation, physical address information may also be written there.

In another preferred embodiment, the highest linear velocity, aboutwhich information is stored in advance on a read-only management area(PIC zone), is 19.7 m/sec or less.

On an optical information storage medium according to the presentinvention, information is written by the method of writing informationon the optical information storage medium described above.

In one preferred embodiment, the optical information storage medium is aBlu-ray disc (which is either a BD-R or a BD-RE) on which informationcan be written at 6× or higher rates.

In another preferred embodiment, a rewritable area or a write-once areainside a BCA, a lead-in zone and/or a lead-out zone of the opticalinformation storage medium includes an area on which information aboutthe highest writable linear velocity Sx and/or information about theradial location Rx is/are written. Optionally, instead of, or inaddition to, the radial location information, physical addressinformation may also be written there. Also, if the optical informationstorage medium satisfies the prescribed range of the reference values atdifferent locations or range from the storage locations of the radiallocation information and/or physical address information (e.g., outsideof the radial location and/or the physical address), then informationabout such different locations or range may be written as informationunique to the optical information storage medium.

The present invention can be used effectively in a method and apparatusfor inspecting an optical information storage medium to get a read/writeoperation done quickly with high densities, an optical informationstorage medium, and a method of writing information. Such an opticalinformation storage medium on which a read/write operation can be donequickly with high densities and its associated recorder and player canbe used effectively in digital consumer electronic appliances andinformation processors.

1. A method for inspecting an optical information storage medium, themethod comprising the steps of: irradiating the optical informationstorage medium with a laser beam and rotating the storage medium by aconstant linear velocity control technique by reference to the radiallocation at which the laser beam forms a spot on the storage medium;changing the rotational velocities according to the radial location onthe storage medium between at least two linear velocities that include afirst linear velocity Lv1 and a second linear velocity Lv2, which ishigher than the first linear velocity Lv1; generating a focus errorsignal and/or a tracking error signal based on the light reflected fromthe storage medium; performing a focus control and/or a tracking controlon the laser beam that irradiates the storage medium based on the focuserror signal and/or the tracking error signal; and passing branchedoutputs of the focus error signal and/or the tracking error signalthrough predetermined types of frequency band-elimination filters forthe focus and/or tracking error signal(s) to obtain residual errors ofthe focus and/or tracking error signal(s) and comparing the residualerrors to predetermined reference values, wherein a gain crossoverfrequency of the focus control remains the same, no matter whether theoptical information storage medium, being subjected to the focus controlto make a comparison to the predetermined reference value, is rotated atthe first linear velocity or the second linear velocity, and wherein again crossover frequency of the tracking control also remains the same,no matter whether the optical information storage medium, beingsubjected to the tracking control to make a comparison to thepredetermined reference value, is rotated at the first linear velocityor the second linear velocity.
 2. A method of reading information fromthe optical information storage medium to be inspected by the method forinspecting an optical information storage medium according to claim 1,the method comprising the steps of: irradiating the optical informationstorage medium with light and rotating the optical information storagemedium; and reading information from the optical information storagemedium.